The low coherence optical interferometry has been proven to be an effective tool for characterization of thin and ultra-thin semiconductor wafers and other materials. It is particularly valuable for the measurement of the thickness of wafers thinner than 150 μm, or wafers mounted on dielectrics materials such as tapes or sapphire plates. For these applications, standard well-established non-contact thickness gauges, such as air pressure or capacitance gauges do not provide direct physical results which meet industry process windows or require introduction of additional experimental parameters. While the bulk of effort was concentrated in the area of metrology for manufacturing of ultra thin Silicon wafers, other very promising areas include metrology of III-V materials mainly for opto-electronics and microwave applications and metrology of micro electro mechanical (MEM) structures.
It has been recognized that low coherence optical interferometry can be used to measure absolute distances between a probe and a wafer. The accurate distance ranging measurements are necessary when measuring physical characteristics of the wafer such as bow and warp. In practice, the absolute distance ranging measurements were not very accurate due to thermal drift of the optical elements of the system. The present invention reduces this effect, and in particular eliminates the influence of the thermal drift of the fiber optic components on performance of the low coherence optical interferometer.